1. Field of the Invention
The invention relates to an absorbent fibrous sheet material made from two or more webs plied together and to a method and apparatus for forming it. Such webs are preferably made from papermaking fibers by conventional papermaking techniques and are useful for sanitary paper products such as towels and other wiper products.
2. Brief Description of the Prior Art
A strong trend exists in the paper industry to develop disposable products from papermaking fibers to serve as substitutes for conventional cloth products which are used as wipers and towels in both the home and industrial shops. To successfully gain consumer acceptance of these paper products, they must closely simulate cloth in both consumer perception and in performance. Thus, certain physical properties must be present in a successful product. Among these properties are softness, strength, stretchability, absorbency, ability to wipe dry, and bulk. Bulk is particularly important not only because it enables the paper product to feel like cloth, but also because it favorably interrelates with other desirable properties, such as softness and absorbency. And, of course, absorbency is very important where the product is to be used to wipe up spills.
Probably the most commonly employed method to produce a wiperlike paper product having the desirable bulk and absorbency is to laminate two or more embossed conventional paper webs together. One such method which has been found particularly advantageous is disclosed in U.S. Pat. No. Re 27,453. The "27,453 method" employs a pair of meshed embossing rolls which mechanically work two webs passed through the nip formed by the two rolls to produce crests and depressions in the webs and join them together by mechanical working of the webs at locations intermediate the crests and depressions. The "27,453 method" offers many advantages, one of which is an efficient web joining process due to the web working being performed on the sides of the embossing knuckles and involving shearing action in addition to compression against the webs. Another advantage is that the shearing action can be employed to perforate the webs and thus increase stretch and softness due to flexibility of the product. One disadvantage in the "27,453 method" is that the two webs are nested together as a result of being embossed together without spaces between the two webs. The addition of spaces between two webs would be beneficial because it creates additional bulk and absorbency in the product, as well as increased softness due to compressibility.
There are methods of laminating two or more webs together which avoid nesting the two webs together. In those methods, the two webs are embossed separately to produce crests and depressions on the webs and then brought together after applying adhesive to the crests on at least one of the webs. The two webs are joined together in a non-nested manner, which provides spaces between the two webs to increase the absorbency and bulk of the product.
In one such example of the prior art, disclosed in U.S. Pat. No. 3,414,459, the crests of each web on the sides facing each other are joined together with an adhesive. In another example of the prior art, disclosed in U.S. Pat. No. 3,556,907, the embossed configuration of each web is different from the other web, and the crests on one web are joined with adhesive to the depressions of the other web. In yet another example of the prior art, disclosed in U.S. Pat. No. 3,708,366, the height of the crests of the first web is greater than the height of the crests on the second web, and the higher crests on the first web are joined with adhesive to the depressions on the second web.
All of the prior art examples of two-ply laminated webs where the webs are embossed before being joined together generally require the use of adhesive, which results in a harsher product, a more complex process, and additional expense. There is some suggestion in the prior art that the webs can be joined by the application of moisture and pressure to the webs at their crests to compress the webs together, but such pressure would have to be excessive to provide satisfactory joining strength, if even possible. On the other hand, the "27,453 method", while avoiding the described disadvantages of the other plying methods, does not create the amount of bulk and absorbency in the product which can be created in the non-nested products. Practice of the present invention combines the advantages of the joining technique and increased stretch and softness due to flexibility of the "27,453 method" with the increased bulk and absorbency and softness due to compressibility of the non-nested two-ply prior art methods.